Oxidation process for preparing 3-formyl-cephem derivatives

ABSTRACT

The present invention relates to an improved process for oxidizing 3-hydroxy-methyl-cephem derivatives to the corresponding 3-formyl-cephem derivatives. In particular this oxidation process is for the preparation of 7-[2-(5-amino-[1,2,4]thia-diazol-3-yl)-2-hydroxyimino-acetylamino]-3-formyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid derivatives of formula (I) using a combination of a hypervalent iodine oxidizing agent of the type 10-I-3 such as bis(acetoxy)iodo-benzene (BAIB) and a catalyst such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). These compounds of formula (I) are intermediates in the synthesis of ceftobiprole.

The present invention relates to an improved process for oxidizing 3-hydroxy-methyl-cephem derivatives to the corresponding 3-formyl-cephem derivatives. In particular this oxidation process is for the preparation of 7-[2-(5-amino-[1,2,4]thia-diazol-3-yl)-2-hydroxyimino-acetylamino]-3-formyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid derivatives of formula (I) using a combination of a hypervalent iodine oxidizing agent of the type 10-I-3 such as bis(acetoxy)iodo-benzene (BAIB) and a catalyst such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). These compounds of formula (I) are intermediates in the synthesis of ceftobiprole.

Compounds of formula (I) and methods for preparing these compounds have been disclosed in WO-01/90111. WO-01/90111 also discloses a process for the preparation of ceftobiprole. Other methods for the preparation of ceftobiprole are disclosed in WO-99/65920 and Drugs of the Future, 30(1), p. 11-22 (2005).

Ceftobiprole is a parenterally administered cephalosporin with high affinity for most penicillin-binding proteins, including the mecA product penicillin binding protein (PBP) 2a, rendering it active against methicillin-resistant Staphylococcus aureus (MRSA). Ceftobiprole shows broad-spectrum activity against relevant resistant Gram-positive and Gram-negative pathogens in vitro and has a low liability to induce resistance. It is administered in vivo as a water soluble prodrug, ceftobiprole medocaril, which is rapidly cleaved in plasma to form ceftobiprole, diacetyl and CO₂. The chemical structure of ceftobiprole and ceftobiprole medocaril are shown below.

WO-01/90111 discloses two oxidation procedures (Example 2 on page 16) for oxidizing 3-hydroxy-methyl-cephem derivatives of formula (II) to the corresponding 3-formyl-cephem derivatives of formula (I):

The first oxidation procedure disclosed in WO-01/90111 uses a mixture of an inorganic hypohalite such as sodium hypochlorite and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) whereby the reaction mixture is a two-phase mixture of water and dichloromethane that is vigorously stirred. Substituent R¹ is a hydroxy protecting group (triphenylmethyl group) and R² is a carboxylic acid protecting group, (diphenylmethyl). The reported yield is 74%.

The second oxidation procedure disclosed in WO-01/90111 uses MnO₂ as the oxidizing agent suspended in a mixture of tetrahydrofuran and dichloromethane. The reported yield is 52%.

In general the present invention concerns the oxidation of a primary alcohol to an aldehyde using an oxidizing agent. A number of art known oxidizing agents are known such as Jones reagent (chromic acid and sulfuric acid in water), Collins reagent (dipyridine Cr(VI) oxide), Dess-Martin periodinane, pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), MnO₂, o-iodoxy benzoic acid (IBX), methyl-2-iodoxy benzoate, isopropyl-2-iodoxy benzoate,trichloro isocyanuric acid, and a combination of TEMPO with an inorganic hypochlorite. The selection of the most suitable oxidizing agent is a cumbersome process wherein issues such as over-oxidation to a carboxylic acid, yield, impurities, cost, reaction time, scale-up possibilities, and the like have to be evaluated in order to achieve the best results.

The oxidation procedures to convert 3-hydroxymethyl-cephem derivatives of formula (II) to the corresponding 3-formyl-cephem derivatives of formula (I) as disclosed in WO-01/90111 (Example 2 on page 16) have the following disadvantages:

-   -   low yield: 52% when MnO₂ is used     -   low yield: 74% when a combination of sodium hypochlorite and         TEMPO as oxidizing agent is used     -   sodium hypochlorite needs to be carefully dosed in a continuous         way to minimise over-oxidation (formation of S-oxides)     -   heterogeneous two-phase system of water and dichloromethane that         needs to be stirred vigorously     -   large volume of solvent is needed: about 7.6 litre/mol.

It has now been found that the above mentioned disadvantages for the oxidation of 3-hydroxymethyl-cephem derivatives of formula (II) to the corresponding 3-formyl-cephem derivatives of formula (I) can be overcome when using a combination of a hypervalent iodine oxidizing agent of the type 10-I-3 such as bis(acetoxy)iodobenzene (BAIB) and a catalyst as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). This oxidation system uses a catalytic amount of TEMPO ranging from 0.05 mol to 0.2 mol and a slight excess of BAIB in an amount from 1 mol to 1.2 mol. BAIB regenerates the TEMPO that is being consumed during the oxidation reaction in order to close the catalytic cycle. This oxidation procedure has the following advantages:

-   -   improved yield : up to 84%     -   improved purity ranging from 92.8 to 97.9% (purity measured         using LC), compared to a purity of 83 to 92% when the above         mentioned procedure from the prior art was reproduced in-house     -   no issues with addition rate of the oxidizing agent and no         over-oxidation, thereby resulting in higher product quality     -   one organic phase and therefore no need for vigorous stirring,         therefore easier and more robust to scale-up     -   lower volume of solvent used: 2 to 5 litre/mol.

The present invention concerns a process for the preparation of a 7-[2-(5-amino-[1,2,4]thiadiazol-3-yl)-2-hydroxy-imino-acetylamino]-3-formyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid derivative of formula (I)

wherein R¹ is a hydroxy protecting group and R² is a carboxylic acid protecting group, which process is characterized in that it comprises oxidizing a compound of formula (II)

with a combination of a hypervalent iodine oxidizing agent of the type 10-I-3 such as bis(acetoxy)iodo-benzene (BAIB) or [bis(1,1,1-trifluoroacetoxy)iodo]benzene (BTIB) and a catalyst selected from 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy and 4-(acetylamino)- 2,2,6,6-tetramethyl-1-piperidinyloxy in a suitable solvent.

The term “hydroxy protecting group” as used herein defines a protecting group which is generally used to replace a hydrogen of the hydroxyl group. Examples of such a group is e.g. benzyl, phenylethyl, naphthalenylmethyl, triphenylmethyl, or a tri(C₁₋₆alkyl)silyl such as e.g. trimethylsilyl, or tert-butyl-dimethylsilyl. A commonly used hydroxy protecting group is a triphenylmethyl group (also called a trityl group).

The term “carboxylic acid protecting group” as used herein defines a protecting group which is generally used to replace a hydrogen of the carboxyl group. Examples of such a group is e.g. diphenylmethyl, tert-butyl, p-nitrobenzyl, p-methoxy-benzyl, methoxymethyl and the like. Diphenylmethyl is a commonly used carboxylic acid protecting group.

The term “C₁₋₆alkyl” as used herein defines straight and branched chain saturated hydrocarbon radicals having from 1 to 6 carbon atoms such as, for example, methyl, ethyl, propyl, butyl, 1-methylethyl, 2-methylpropyl, 2-methylbutyl, pentyl, hexyl and the like.

Hypervalent iodine oxidizing agent of the type 10-I-3 have been described by De Mico A. et al. in J. Org. Chem., 62, 6974-6977 (1997) such as bis(acetoxy)iodo-benzene (BAIB) or [bis(1,1,1-trifluoroacetoxy)iodo]benzene (BTIB).

The amount of catalyst selected from 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy and 4-(acetylamino)-2,2,6,6-tetramethyl-1-piperidinyloxy ranges between 0.05 and 0.2 mol with respect to the compound of formula (II) and the amount of hypervalent iodine oxidizing agent of the type 10-I-3 ranges between 1 mol and 1.2 mol with respect to the compound of formula (II). In practice the amount of catalyst is usually 0.1 mol and the amount of hypervalent iodine oxidizing agent of the type 10-I-3 is 1.1 mol with respect to the compound of formula (II).

Suitable solvents for use in the oxidation process of the present invention are selected from halogenated hydrocarbons such as dichloromethane; esters such as ethyl acetate; ethers such as tetrahydrofuran; hydrocarbons such as toluene; polar solvents such as acetone and acetonitrile; and solvent mixtures thereof, such as solvent mixtures of dichloromethane with tetrahydrofuran, acetonitrile, or ethylacetate, solvent mixtures of ethyl acetate with tetrahydrofuran, and solvent mixtures of toluene with tetrahydrofuran.

Experimental Part

General Oxidation Procedure

-   -   1 mol of         7-[2-(5-amino-[1,2,4]thiadiazol-3-yl)-2-trityloxyimino-acetylamino]-3-hydroxymethyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic         acid benzhydryl ester (compound 2) was added to a reaction         vessel     -   solvent was added and cooled to 10° C.     -   0.10 mol of catalyst selected from         2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO),         4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy and         4-(acetylamino)- 2,2,6,6-tetramethyl-1-piperidinyloxy was added     -   stirred for another 5 minutes     -   1.1 mol of a hypervalent iodine oxidizing agent of the type         10-I-3 selected from bis(acetoxy)iodo-benzene (BAIB) was added     -   stirred till complete conversion (as measured by LC)     -   work-up procedure:         -   the reaction product was precipitated by the addition of an             anti-solvent such as cyclohexane (other suitable             anti-solvents are methylcyclohexane, isooctane,             diisopropylether and cyclopentylmethylether)         -   precipitate was filtered off         -   precipitate was washed         -   isolated             7-[2-(5-amino-[1,2,4]thiadiazol-3-yl)-2-trityloxyimino-acetylamino]-3-formyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic             acid benzhydryl ester (compound (1)) was dried at 30° C.             under vacuum     -   Results: catalyst is TEMPO         (2,2,6,6-tetramethyl-1-piperidinyloxy) and hypervalent iodine         oxidizing agent of the type 10-I-3 is BAIB

TEMPO BAIB purity of isolated yield solvent # eq. # eq compound 1 mol % CH₂Cl₂ (10 L/mol)  0.1 mol  1.1 mol 95.6% 81.1 CH₂Cl₂ (5 L/mol)  0.1 mol  1.1 mol 96.1% 81.9 ethylacetate (5 L/mol)  0.1 mol  1.1 mol 95.3% 58.2 THF (2 L/mol)  0.1 mol  1.1 mol 96.3% 72.7 acetonitril (3 L/mol)  0.1 mol  1.1 mol 97.9% 64.2 CH₂Cl₂/THF  0.1 mol  1.1 mol 93.2% 83.9 (2 L/mol/1 L/mol) CH₂Cl₂/acetonitril 0.05 mol  1.1 mol 95.6% 81.7 (2.5 L/mol/0.5 L/mol) CH₂Cl₂/acetonitril 0.05 mol 1.05 mol 92.8% 78.1 (2.25 L/mol/0.75 L/mol) 

1. A process for the preparation of a 7-[2-(5-amino-[1,2,4]thiadiazol-3-yl)-2-hydroxy-imino-acetylamino]-3-formyl-8-oxo-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylic acid derivative of formula (I)

wherein R¹ is a hydroxy protecting group and R² is a carboxylic acid protecting group, which process is characterized in that it comprises oxidizing a compound of formula (II)

with a combination of a hypervalent iodine oxidizing agent of the type 10-I-3 and a catalyst selected from 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy and 4-(acetylamino)- 2,2,6,6-tetramethyl-1-piperidinyloxy in a suitable solvent.
 2. The process as claimed in claim 1 wherein the hypervalent iodine oxidizing agent is selected from bis(acetoxy)iodobenzene (BAIB) or [bis(1,1,1-trifluoroacetoxy)-iodo]benzene (BTIB).
 3. The process as claimed in claim 2 wherein the hypervalent iodine oxidizing agent is bis(acetoxy)iodobenzene (BAIB).
 4. The process as claimed in claim 3 wherein the hydroxy protecting group R¹ is selected from benzyl, phenylethyl, naphthalenylmethyl, triphenylmethyl, or a tri(C₁₋₆alkyl)silyl, and the carboxylic acid protecting group R² is selected from diphenylmethyl, tert-butyl, p-nitrobenzyl, p-methoxybenzyl, methoxymethyl.
 5. The process as claimed in claim 4 wherein R¹ is triphenylmethyl and R² is selected from diphenylmethyl.
 6. The process as claimed in claim 5 wherein the catalyst is 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO).
 7. The process as claimed in any one of claims 5 wherein the amount of catalyst ranges between 0.05 and 0.2 mol with respect to the compound of formula (II) and the amount of BAIB ranges between 1 mol and 1.2 mol with respect to the compound of formula (II).
 8. The process as claimed in claim 7 wherein the amount of catalyst is 0.1 mol and the amount of BAIB is 1.1 mol with respect to the compound of formula (II).
 9. The process as claimed in claim 8 wherein the suitable solvent is selected from halogenated hydrocarbons, esters, ethers, hydrocarbons, polar solvents, and solvent mixtures thereof.
 10. The process as claimed in claim 9 wherein the suitable solvent is selected from dichloromethane, ethyl acetate, tetrahydrofuran, toluene, acetone, and acetonitrile, and solvent mixtures thereof.
 11. The process as claimed in claim 10 wherein the suitable solvent is a solvent mixture selected from solvent mixtures of dichloromethane with tetrahydrofuran, acetonitrile or ethylacetate; solvent mixtures of ethyl acetate with tetrahydrofuran; and solvent mixtures of toluene with tetrahydrofuran.
 12. The process as claimed in any one of claims 6 wherein the amount of catalyst ranges between 0.05 and 0.2 mol with respect to the compound of formula (II) and the amount of BAIB ranges between 1 mol and 1.2 mol with respect to the compound of formula (II). 